Armoured cable for down hole electrical submersible pump

ABSTRACT

An ESP is suspended downhole on a cable comprising spirally wound copper clad steel conductors surrounded by nonmetallic sheathing wound in an opposite spiral direction, preferably in the direction of rotation of the rotor of the ESP. The torque exerted on the cable by the ESP is reacted against the conductors, relieving stress from the sheathing so that the cable may be light in weight yet support substantial loads.

This invention relates to electrical submersible motor driven tools, particularly pumps for use in hydrocarbon wells, and to the cables by which they are supplied with electrical power from the surface.

An electrical submersible pump (ESP) is an assembly comprising an electrical motor driving a pump for pumping fluids from the wellbore to the surface. The pump may comprise a rotor, e.g. a centrifugal impeller, or a reciprocating or other type of pumping element. The motor is generally a three phase motor supplied via a cable comprising three conductors.

ESPs for hydrocarbon wells are typically very long and heavy and are deployed at great depths. Where the cable is used to suspend the ESP during deployment and retrieval, it must have sufficient tensile strength to support its own weight together with the weight of the ESP and transient forces resulting from friction between the ESP and the wellbore. Alternatively, the ESP can be deployed on a wireline or on continuous (coiled) tubing (CT) which supports the cable. Alternatively, the motor can be suspended on the cable and deployed separately from the pump, the motor module being mechanically connected to the pump module downhole in the deployed position.

Copper conductors have insufficient tensile strength to support their own weight at great depths. Therefore copper conductors are typically encased in one or more layers of spirally wound high tensile steel wire armour which mechanically supports the weight of the cable and the ESP attached to it. Each layer may be wound in an opposite spiral direction to the next, so as to react the torque developed by the tendency of each layer to straighten under tensile load. The armour protects the conductors against abrasion but adds substantially to the weight of the cable, and also tends to make the cable much less flexible. This in turn causes difficulty when deploying the cable, requiring more expensive equipment with larger diameter sheaves, while bending can cause kinking and damage to the armour.

Alternatively, it is known to encase the conductors in a welded tubular steel casing. It is also known to provide solid steel conductors, each having a copper cladding, whereby the cladding carries most of the current and is supported by the steel core.

It is the object of the present invention to provide an assembly comprising an electrical submersible motor, particularly for an electrical submersible pump assembly, supported on a cable which is more conveniently deployed in downhole applications.

In accordance with the present invention there is provided an assembly as defined in the claims.

The invention provides a cable that is advantageously light in weight and yet capable of suspending a motor or ESP. The tensile load is supported primarily by the copper clad steel conductors. The nonmetallic sheathing is more flexible but also much lighter in weight than conventional steel armour. Surprisingly it is found that despite its flexibility, the nonmetallic spiral sheathing wound in an opposite direction to the self supporting copper clad steel conductors adequately reacts the torque generated by the conductors under tensile load, whereby the cable is prevented from untwisting even when loaded by a motor or ESP suspended at great depth, for example, 1 km, 2 km or even further below the surface.

Preferably, the motor is a permanent magnet motor, which is substantially lighter than a conventional, induction motor of similar power, and therefore exerts less tensile load on the cable, which in turn reduces the torque generated by the conductors during deployment and by the static weight of the motor.

In other embodiments the motor need not drive a pump or compressor. Advantageously, the motor may be deployed as a module separately from the pump or other load, which further reduces the tensile load on the cable.

Further features and advantages will be understood from the following description of an illustrative embodiment which is set forth, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which:

FIG. 1 is a side view of production tubing installed in a borehole;

FIG. 2 shows an ESP;

FIG. 3 is a cross section through a first electrical cable;

FIG. 4 is a cross section through one of the conductors of the first cable; and

FIG. 5 shows the first cable attached to the upper end of the ESP.

Corresponding features are indicated by the same reference numerals in each of the figures.

Referring to FIGS. 1 and 2, there is shown a well completion with casing 1 cemented into the wellbore. A packer 2 with elastomer seals 9 and including a polished bore receptacle (PBR) 3 is set in the casing. The production tubing 4 stings into the PBR with a stinger 5 and seal 6.

A no go landing feature 8 is included to provide a reference stop point when installing the ESP 50. Beneath the no go landing feature 8 is located a sub surface safety valve 11 controlled by a control line 13. In other embodiments, the sub surface safety valve may not be provided.

The electric submersible pump (ESP) 50 consists of a stinger and pump inlet 64, a rotary pump or compressor 66 with a rotating impeller 66′ and a pump outlet 69, a motor seal 70, a permanent magnet motor 67 with a rotor 67′, and a sensor package 61. The ESP is deployed on an umbilical connection comprising a self supporting electrical cable 71, which is to say, a cable capable of supporting its own weight when deployed downhole.

The ESP 50 is suspended and lowered down the well on the umbilical 71, and comes to rest on the landing feature 8, with the stinger 64 forming a seal against the inner surface of the production tubing 4. The surface operator can detect when the ESP has reached this point (for example by monitoring the weight on the wireline or the length of wireline deployed). In alternative embodiments, the motor 67 may be suspended from the umbilical 71 and deployed separately from the pump, and mechanically connected to the pump in the deployed position.

As well as suspending the motor during deployment, the umbilical 71 supplies the ESP's motor 67 with electric power. The ESP can be operated from the surface, the motor 67 driving the pump 66 so that well fluid from beneath the pump inlet 64 is drawn up through the pump 66, and exits through the pump outlet 69 and up through the production tubing 4 to the surface.

Referring to FIGS. 3 and 5, the cable 71 comprises a group of three insulated metallic conductors 161 arranged in a triangular formation and wound in a spiral configuration in a first direction of rotation D1. The conductors are surrounded by an insulating filler 162 which may, for example, be extruded around the conductive cables. (For clarity, the filler 162 and details of each individual conductor are not shown in FIG. 5.) The conductors and filler are enclosed in a spirally wound casing comprising an outer sheathing of non-metallic fibres 164 such as aramid or para-aramid fibres, e.g. Kevlar®, or other plastics fibres having suitable tensile strength and resistance to wellbore fluids as known to those skilled in the art. The fibres 164 are wound in a spiral configuration in a second direction of rotation D2 opposite to the first direction D1, so that they act in tension to oppose the torque generated by the tendency of the conductors to straighten under tensile load.

The weight of the cable is supported by the conductors 161. The filler 162 and the non-metallic fibres 164 do not themselves provide any significant loadbearing characteristics. The fibres 164 do however protect the body of the cable from damage from friction or pressure from other components as it is deployed down the well. Preferably, the casing (outer sheathing) does not include any metallic armour, so that the conductors (and, where present, the individual tubular steel casing 169 around each individual conductor) comprise the only metallic content of the cable. This provides a lightweight construction which is able to bear surprisingly large loads.

Referring to FIG. 4, each conductor comprises a central steel core 168 clad in a copper layer 167, which is coated in a primary insulator 166 (for example kapton Tape®) having a high dielectric coefficient, and a secondary insulator 165 which can provide mechanical protection, and a further metal layer, such as a stainless steel layer 169 around the secondary insulator 165. This layer is seam welded and is a snug fit around the insulation 165. The additional stainless steel layer 169 may not always be required, but can be used to provide a second conductive path in the conductor 161, for telemetry or separate power for sensor systems, or a shielding layer to reduce the electrical noise from the power cable. Also, each conductive element could be stranded or further comprised of a plurality of steel conductors each clad with a copper layer.

The motor comprises a rotor 67″ which rotates in a direction D3 relative to the stator 67′″, so that the rotor torque is reacted against the stator and casing of the motor, which in turn exert torque on the cable in an opposite direction D4 to the rotation of the rotor. It is found that by arranging the rotor to rotate in use in the same direction as that of the spiral winding of the nonmetallic sheathing—which is to say, in the direction in which the end of the sheathing attached to the ESP would be rotated in order to tighten its spiral windings—the torque exerted by the stator and motor casing against the cable is effectively reacted against the conductors, being in the same direction D4 as that D1 of the spiral winding of the conductors, i.e. in that direction in which the end of the conductors attached to the ESP would be rotated in order to tighten their spiral winding. In this way the torque exerted on the cable by the rotation of the rotor in use is arranged to relieve stress on the nonmetallic sheathing while the motor is in use. Since the sheathing is not required to resist torque applied by the rotation of the tool in use, metallic sheathing is not required; the use of lighter non-metallic sheathing enables the cable to support relatively larger loads at relatively greater depths, and also makes the cable more flexible and hence easier to deploy with smaller diameter sheaves, for example, via a lubricator.

Table 1 shows a comparison of weights and lengths of a permanent magnet motor versus those of an induction motor ESP.

TABLE 1 5.4″ Permanent Magnet Motor ESP System 5.4″ Induction Motor ESP System Item Length Wt Item Length Wt Umbilical 1.16″ dia 1.2 lb/ft Umbilical 1.16″ dia 1.2 lb/ft Connector 1.0 ft 10 lbs Connector 1.0 ft 10 lbs Sensor sub 3.0 ft 60 lbs Sensor sub 3.0 ft 60 lbs PM Motor 250hp 7.6 ft 515 lbs Induction Motor 250hp 39.8 ft 2574 lbs Seal 8.9 ft 385 lbs Seal 8.9 ft 385 lbs Pump discharge 0.8 ft 23 lbs Pump discharge 0.8 ft 23 lbs Pump 20.4 ft 924 lbs Pump 20.4 ft 924 lbs Thrust bearing 8.9 ft 385 lbs Thrust bearing 8.9 ft 385 lbs Stinger and gauge 3.5 ft 50 lbs Stinger and gauge 3.5 ft 50 lbs Overall assembly 54.1 ft 2352 lbs Overall assembly 86.3 ft 4411 lbs

Referring to table 1, it will be seen that a permanent magnet motor is significantly lighter and shorter than a conventional induction motor of similar power, significantly reducing the hanging weight and making it possible to deploy it via a lubricator. A conventional ESP system, using an 250 hp induction motor, typically has a have a weight of 4411 lbs (2001 kg) and a length of 86.3 feet (26.3 metres). The induction motor contributes a weight of 2574 lbs (1168) and 39.8 feet (12.1 metres). In contrast, a 250 hp permanent magnet motor of the same diameter may typically have a weight of 515 lbs (234 kg) and 7.6 feet (2.3 metres). The total weight and length of an ESP using a permanent magnet motor is therefore 2352 lbs (1067 kg) and 54.1 feet (16.5 metres).

Usually, assemblies over 60 feet (18.2 metres) present difficulties for injecting into a well bore, and without special equipment the well must be killed before the equipment is introduced. The shorter length of the permanent magnet motor, particularly where the motor is deployed as a separate module, means that the ESP may fit into a lubricator by means of which it may be conveniently injected into a live well. This is assisted by the greater flexibility of the novel cable.

Advantageously, the umbilical can be made thin enough, and therefore flexible enough to pass over sheaf wheels rather than being injected into the well using larger equipment such as a CT injector. This advantage is particularly realised where the motor is a permanent magnet motor which is short enough to fit into a lubricator.

In summary, a preferred embodiment provides an ESP which is suspended downhole on a cable comprising spirally wound copper clad steel conductors surrounded by nonmetallic sheathing wound in an opposite spiral direction, preferably in the direction of rotation of the rotor of the ESP. The torque exerted on the cable by the ESP is reacted against the conductors, relieving stress from the sheathing so that the cable may be light in weight yet support substantial loads.

Those skilled in the art will readily conceive further adaptations, and it will be understood that the invention is limited only by the scope of the claims. 

1. An assembly comprising an electrical submersible motor suspended on a self-supporting cable; the cable comprising a group of insulated metallic conductors wound in a spiral configuration in a first direction of rotation and enclosed in a spirally wound casing; wherein each of the conductors comprises a steel core clad with copper; and wherein the casing comprises non-metallic fibres wound in a spiral configuration in a second direction of rotation opposite to the first direction.
 2. An assembly according to claim 1, wherein the motor is a permanent magnet motor.
 3. An assembly according to claim 2, wherein the motor drives a pump or compressor.
 4. An assembly according to claim 3, wherein the pump or compressor is a rotary pump or compressor.
 5. An assembly according to claim 4, wherein the motor comprises a rotor which rotates in use in the second direction.
 6. (canceled)
 7. An assembly according to claim 3, wherein the motor comprises a rotor which rotates in use in the second direction.
 8. An assembly according to claim 2, wherein the motor comprises a rotor which rotates in use in the second direction.
 9. An assembly according to claim 1, wherein the motor drives a pump or compressor.
 10. An assembly according to claim 9, wherein the pump or compressor is a rotary pump or compressor.
 11. An assembly according to claim 10, wherein the motor comprises a rotor which rotates in use in the second direction.
 12. An assembly according to claim 9, wherein the motor comprises a rotor which rotates in use in the second direction.
 13. An assembly according to claim 1, wherein the motor comprises a rotor which rotates in use in the second direction. 